LearnCAx Blog 3482 Electrostatic Precipitators Esp Analysis Using Cfd

8/10/2019 LearnCAx Blog 3482 Electrostatic Precipitators Esp Analysis Using Cfd

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Electrostatic Precipitators (ESP) Analysis Using CFD

Subhransu Majhi, Swapnil Dindorkar & Ganesh Visavale 1 LearnCAx, CAx education division

Centre for Computational Technologies Pvt. Ltd. , Pune, India

Note : Videos used in this blog (if any) are not included in this document. Please visit theoriginal blog to view the videos.

The particulate emission from process industries has received great attention due to theupcoming strict environmental protection agency (EPA) regulations and conservation inrecent years. The Electrostatic precipitators (ESP) since its development in 1907 byFrederick G. Cottrell (Professor of chemistry at the University of California, Berkeley) havebeen the most common, effective and reliable technologies for removal of hazardousemissions like flue gases, acid droplets and fine particles.

The industrial ESP are capable of handling large gas volumes with a wide range of inlettemperatures, pressures, dust volumes and gas conditions and exhibit complex interactionmechanism between electric field, fluid flow and particulate flows.

Figure 1 : Electrostatic precipitators

1 Dr. Ganesh Visavale ([email protected] ), LearnCAx, Centre for Computational Technologies Pvt. Ltd
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Objective of study : To separate out maximum particulate pollutants with the minimumamount of power input.

From the velocity of the particles, we can find out the kinetic energy they possess. Based

on this force, we need to generate an electrostatic energy (from the electrodes inside thedevice) of a higher magnitude than the kinetic energy of the particles in order tosuccessfully remove them out of the flow. As the target is to minimize the energyrequired, it is obvious that we reduce the velocity of the flow as much as possible. It isequally important to have uniform & optimum velocity throughout the device. This means,velocity should not exceed the threshold value anywhere inside the device. If velocityexceeds the max permissible value then, particles will not get trapped and hence escapeinto atmosphere. At the same time if at some place, the velocity is too low then theparticles will get collected at the beginning of the device itself and the energy supplied tothe electrode sheets which are towards the end of the device will just go wasted. Hence itis very essential to maintain uniform velocity throughout the device.

In order to maintain uniform velocity one needs to have a good design of the device aswell as entry and exit. Here CFD plays a very crucial role in testing and validating thedesigns.

Electrostatic Precipitator Device :

A simple demo case presented here was designed on the basis of space availability andhence it is quite visible in the geometry image (see below) that this design is not anoptimized one. There is an expected flow-separation at the inlet which is easily visible,but in order to predict this designs ineffectiveness a complete CFD simulation needs to bedone and based on the results the design will be optimized and another CFD study isconducted on the modified design. The CFD study will contain a very basic flow predictioninside the device. The discrete pollutants being so light in weight, that they wont beinfluencing the flow behavior at all. Hence, the discrete pollutant phase is not modeled inthis study and instead just air is modeled. The velocity prediction would give theunderstanding of the effectiveness of the device.

Figure 3: Geometry of a typical ESP
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A steady-state single phase simulation was carried out with Reynolds-averaged Navier Stokes equations coupled with the k-epsilon turbulence model equations. Defaultconvergence criteria of 10-3 for all the equations were considered.

CFD Results : Let us have a look at the analysis data. The images below clearly show the non-uniformityin the velocity distribution inside the ESP Device.

Figure 4: Non-uniform flow distribution in ESP

Figure 5: Velocity Contours


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In this setup, the available power was just sufficient enough to separate the particles(from the bulk flow) having velocities of 1.5 m/s or below. The following images willexplain it in detail.


In the above images, can be clearly seen that in the colored zone the velocity is withinrange and the centre part (i.e. unfilled area) indicates that the velocity is not inoperational range of the device. The percentage of area falling within the operationalrange is about 38% and 10% in horizontal and vertical cross-section respectively. Thismeans that only around 25% pollutants would be removed from the exhaust gases whereasthe energy supplied to it was sufficient enough to remove them completely. This gives anidea about how important role a proper design plays in each and every engineeredproduct.

Non-uniformity in velocity distribution was observed within the device due to the suddenexpansion of the duct which caused a flow-separation at the divergence section of theinlet. It would have been much more uniform if the increase of cross sectional area wasdone gradually, however though at times the decision is based on purely non-technicalfactors like space availability & others.

Optimization study :

In the existing design, a steel wire-frame (filter) was introduced at the entrance of theESP to assist in the uniform distribution of flue gases.


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Figure 7: Geometry with Filters

The CFD approach was kept same as that of baseline study performed earlier, except thefilter section that was modeled as a porous zone. Let us now analyze the results fromthe simulation study of this modified geometry.

Figure 8: Velocity Contours Vertical Central Plane

Figure 9: Velocity Contours Horizontal Central PlaneThe above images have made it clear that the flow has become much more uniform thanearlier. Now let us analyze it more quantitatively.


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The above images display the area which falls in the optimum working range of the ESP.The percentage area comes out to be 99%. This clearly states that the efficiency of thedevice has increased from 25% to around 95% and above.

Also below is shown the video animation of the flow distribution in a ESP with and without

filter:
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Summary :

The initial design of the ESP was studied with the help of commercial CFD tool ANSYSFluent and after understanding its ineffectiveness, the design was modified by the

addition of a filter. The insertion of filter at the inlet helped delay the flow separation atthe inlet and improved the distribution of velocity in a more uniform pattern around allthe electrode plates thus improving the efficiency of the ESP from 25% to 95%. Hence, CFDhas proven as a very cost effective and useful tool in designing the Electrostaticprecipitator.

References :

Zhengwei Long, Qiang Yao ; Evaluation of various particle charging models forsimulating particle dynamics in electrostatic precipitators Journal of Aerosol Science41 (2010), 702718.

F.J. Gutirrez Ortiz, B. Navarrete, L. Caadas; Dimensional analysis for assessing theperformance of electrostatic precipitators Fuel Processing Technology 91 (2010), 1783 1793.

G. Skodras, S.P. Kaldis, D. Sofialidis, O. Faltsi, P. Grammelis, G.P. Sakellaropoulos;Particulate removal via electrostatic precipitators CFD simulation Fuel ProcessingTechnology 87 (2006), 623 631.

Shah M.E. Haque, M.G. Rasul, A.V. Deev, M.M.K. Khan, N. Subaschandar; Flowsimulation in an electrostatic precipitator of a thermal power plant Applied ThermalEngineering 29 (2009), 20372042.


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C i h L CA1 Akshay Residency 50 Anand Park Aundh Pune 411007 India

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LearnCAx Blog 3482 Electrostatic Precipitators Esp Analysis Using Cfd